thus far I've made:
max height towers connected by bridges just below cloud level.
my friends' houses
an ancient greek themed city in the desert made of sand
a railroad connecting 2 distant cities (easily earned the achievement riding that)
a harbor with a proper wooden boat at my desert city (it's next to the ocean).
a mining facility that goes down to bedrock
underground tree farming facilities
a submerged glass dome (we manually dug down near the ocean, then flooded it, then fixed the water)
an underwater hideout that you have to dive down to reach the door.
The boat is actually submerged with no water pockets (I had to swim down to dredge the bay and lay the keel and hull).
Other plans I have:
An obsidian factory (lava pool, with water source above it and sand or piston mechanism to release the water onto a section of the lava)
build a "proper" neighborhood with streets, sidewalks, etc and every house I've ever lived in or friends' houses that I remember (not just splatting houses next to each other). What better way to build up a city?
a StarGate Command gate room with a working Nether Portal and a switch wired to a piston to release water into it to close the gate remotely from the control booth.
a Star Gate in the wilderness on a proper ramp, just like we see in SG-1 on most planets.
Beaming rings like the Star Gate pyramid ships use to beam troops down on the planet. Fill the rings with falling lava or water, but in the center hide a ladder up to the ship, floating above. Make a secret underground entrance to this ladder, so from the surface, it looks inaccessible as it is currently "in transit"
In another friend's world, we are recreating a port city from one of our D&D campaigns on an artic Seed while on "Normal" difficulty. this simulates the challenge of living and building in such an environment and shapes how and what we build.
In all of our worlds, we don't use the Dupe Glitch (just learned how from YouTube yesterday and failed to get it to work on a test run in the current 1.7.x version). So, effectively, we couldn't Dupe Glitch if we wanted to.